There are numerous types of analytical methods which currently are known for deriving information about materials. Spectroscopy is a well known and general method for analyzing materials. There are a number of types of spectroscopic methods which, in turn, are applicable to certain types of analyses and measurements, and which have advantages and disadvantages.
Presently, there is a need for improvements in the ability to analyze materials, especially in those cases where such analyses need to be quick, efficient, and accurate. Additionally, there is a real need for such analyses for "in-process" situations; that is, directly on-line with respect to the manufacturing or the processing of materials.
Presently, for many materials, there are a variety of generally conventional spectroscopic methods for analyzing the content and other characteristics of the materials. Some of those methods are infrared transmission, diffuse reflectance, photoacoustic, and emission spectroscopies. While generally these methods give satisfactory results, they are deficient because they require selective, and often destructive, sampling of the materials. Some materials (coal, for example) require grinding or pulverizing. The material must often be removed to a remote laboratory location where the testing and equipment requires time and resources to provide the results. Currently, no contemporaneous, non-destructive, on-line infrared analysis is reasonably possible for solid materials including semisolid materials such as flexible or rubber-like materials.
Many of the aforementioned presently used methods also lack much flexibility in their use. While some of the methods do not require destructive sampling such as grinding or pulverizing, they may not be operable for materials of greater than minimal thickness, or for materials of varying thickness. Conventional transmission, reflection, or emission spectroscopies have problems because the optical density of many materials is too high to permit accurate and reliable measurement. That is, upon heating of a sample, such sample strongly reabsorbs the same wavelengths it strongly thermally emits as infrared radiation. When a thick sample is heated, the deep layers of the sample emit strongly at the preferred wavelengths and only weakly at other wavelengths. This deep-layer strong emission at preferred wavelengths, however, is greatly attenuated before leaving the sample since surface layers of the thick sample preferentially abosrb those particular wavelengths and such process is termed "self-absorption". Self-absorption in optically-thick samples causes severe truncation of strong spectroscopic bands and leads to emission spectra which closely resemble black-body emission spectra representative of an optically thick material being heated to a uniform temperature and which contain little spectral structure characteristic of the material being analyzed.
Attempts have been made to solve this self-absorption problem by thinning sample materials. High-quality spectra of free-standing films and thin layers on low-emission substrates are routinely measured. However, this requires selective sampling and processing of the materials being analyzed.
For other types of spectroscopic methods such as photoacoustic spectrometry which are less subject to optical density problems, deficiencies exist in that they are not easily performed on moving streams of solid materials. Thus, there is a real need in the art for an apparatus and method which has the flexibility to be used both for moving and stationary materials; and for materials which may have significant optical densities.
There is a further need for an apparatus and method which does not require the use of additive materials to or processing of the sample materials, and which can analyze non-destructively and remotely. For example, in some spectroscopic methods, the materials must be ground to fine powders and then diluted in a transparent matrix. Of course, any destructive processing or additive procedures would alter the beginning state of the material being analyzed. For an analytical apparatus and method to be used effectively in a production line, any fundamental change in the material must be avoided. For example, if variable-in-size crushed coal were being analyzed on a moving conveyor, no grinding or addition of any substance would be allowed, as the coal could not then be utilized for its intended purpose in its original state.